Scientists and engineers often employ seismic surveys for exploration, archeological studies, and engineering projects. Seismic surveys can provide information about underground structures, including formation boundaries, rock types, and the presence or absence of fluid reservoirs. Such information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons and ores. Oil companies in particular often invest in extensive seismic surveys to select sites for exploratory oil wells.
Conventional seismic surveys employ artificial seismic energy sources such as shot charges, air guns, or vibratory sources to generate seismic waves. The sources, when fired, create a seismic “event”, i.e., a pulse of seismic energy that propagates as seismic waves from the source down into the earth. Faults and boundaries between different formations create differences in acoustic impedance that cause partial reflections of the seismic waves. A seismic sensor array detects and records these reflections for later analysis. Sophisticated processing techniques are applied to the recorded signals to extract an image of the subsurface structure.
Various techniques have been proposed to eliminate the conventional seismic energy sources from this process. These techniques are often termed “passive seismic” or “micro-seismic” imaging. See, e.g., U.S. Pat. Nos. 5,377,104 and 6,920,083; and B. S. Artman, “Passive Seismic Imaging”, Poster S11E-0334, AGU Fall Meeting, December 2003. The proposed techniques generally rely on environmental seismic events such as earthquakes, hydraulic fracturing, drilling operations, or operations of heavy construction equipment. Typically, the proposed techniques employ cross correlation to extract seismic event information, and thereafter the processing can proceed in much the same fashion as conventional seismic survey processing.
Thus existing seismic imaging techniques rely on event-based analysis of wave propagation patterns. Where passive seismic imaging techniques are rendered unsuitable due to the absence of identifiable seismic events, surveyors are required to employ artificial seismic energy sources. Use of such sources can add significant expense to exploratory seismic surveys.